Outline

Objective

Transcranial intraoperative MEP monitoring in patients with lesions near the motor pathways is thought to be a valuable tool for prevention of new postoperative motor deficits. The aim of this study was to investigate its diagnostic facilities in our patients collective.

Methods

Fifty-nine patients, 21 with spinal (e.g. meningioma, ganglioglioma, metastases, spinal stenosis) and 38 with intracranial lesions (e.g. sphenoid wing meningioma, oligoastrocytoma, AVM) were studied. Transcranial stimulation was performed with a high frequency (500 Hz) electrical train stimulation (train of 5, stimulation rate 0.1 Hz, stimulation intensity max. 100mA/400V, pulse width 0.2-0.7msec) by using two subdermal needle electrodes. MEP were recorded from a forearm flexor, the abductor pollicis brevis, the quadriceps femoris and the anterior tibial muscle. Decreasing amplitudes of more than 50%, increasing latencies of more than 10% and increasing stimulus intensities were reported to the surgeon.

Results

MEP recording of upper limbs was possible in 97.9%, of the lower limbs in 85%. Stimulation intensities had a large variability in voltage and amperage (52V to 400V and 51mA to 100mA, resp.). We saw a temporary maximum amplitude reduction of more than 50% in 26 (44%) patients and 22 (37%) needed an increase in stimulation intensity of more than 20%. Eight of these patients deteriorated in motor function postoperatively. Latencies were completely in normal range. No complications like seizures, infections or skin affections were observed. Intraoperative MEP changes had a sensitivity of 88% and and a specifity of 56%. The positive predictive value of MEP changes for postoperative motor funcion deterioration was 27%, and the negative predictive value was 97%.

Conclusions

Transcranial electrical monitoring of MEP is a practicable and safe method. However, there are many events which can cause amplitude changes of MEP independent from surgical manipulations. Although sensitivity is high, this results in a low specificity of MEP monitoring for prediction of postoperative deterioration of motor function.